I do believe you can enter B'Tran at level 1. You just don't get the ability to explore it until after 40.
"My frozen dairy-based confectionery attracts all the males of the species to the facilities. They all agree on it's superiority. Indeed, it is superior to yours. I could teach you the finer details but that would require monetary recompense on your part." -The Milkshake Song: Vulcan Edition
In addition to being at least level 43 to be eligible to do the mission that grants you access to GO, you also have to be at that point in the story progression. If you aren't, you'll have to do more story missions until you are, or skip missions until you get to it.
Foundry Missions: 1) The Source of Power (ST-HSWUBD5TQ) [Federation] 16+ -----=====*****<[ Fleet Recruitment Thread ]>*****=====-----